Patent GB-790576-A (GB)
Title: Improvements in and relating to aerial systems
IPC: H01Q5/49
Filed: 1955-11-23
Abstract: 790,576. Radio aerials. BIRD, G. A. Aug. 21, 1956 [Nov. 23, 1955], No. 33589/55. Class 40 (7). Relates to aerials which are effective at a plurality of frequencies or over a plurality of bands of frequencies simultaneously. According to the invention generally, the aerial incorporates a conductor resonant at a first frequency and associated with a first reactance so as to be resonant at a second frequency, a second reactance being provided to render the first reactance ineffective at the first frequency whereby the aerial is resonant at both the first and second frequency. The principle of the invention is illustrated in Fig. 1 with reference to an aerial array arranged to be effective at 14, 21 and 28 Mc/s. The aerial comprises a driven element 1, a parasitic director 2 and a parasitic reflector 3. The reflector is cut to be naturally effective at 21 Mc/s. and is loaded at its centre by an inductor 6 so as to be effective at 14 Mc/s. An open-circuited quartet-wave stub resonant at 21 Mc/s. is then included across the inductor so as to short it out at this frequency. The element is thereby made effective at both 14 and 21 Mc/s. The stub provides a small amount of capacitive reactance at 14 Mc/s. and is allowed for by providing a smaller inductor. At 28 Mc/s. the loaded reflector is effectively three half wavelengths long, something in excess of the centre half-wavelength being represented by the inductor, whereby the two end sections effectively present two half-wavelength reflectors operating in phase. The director 2 is cut to be effective at 28 Mc/s. and, in a manner similar to the director, is loaded by an inductor 10 so as to be effective at 21 Mc/s. An open-circuited stub 11 resonant at 28 Mc/s. shunts the inductor to render it ineffective at this frequency. The driven element 1 is selected to be effective at 14, 21 and 28 Mc/s. although aperiodic at these frequencies. The element is fed at its centre by a high-impedance open-wire twin feeder selected to give the lowest average standing wave ratio at the three frequencies at which the aerial is effective. The Specification includes detailed dimensions for the array and in Fig. 3 (not shown) illustrates a practical construction. A mast supports a horizontal boom on the underside of which three channel-shaped members support and protect stand-off insulators which secure the elements of the array. The inductors comprise lumped inductors in the form of wire or tubing which may be protected by polythene covers. The stubs comprise lengths of low impedance twin feeder which are housed within the boom for protection. An aerial coupling unit for use between the twin feeder and a low impedance feeder is described, Fig. 2 (not shown). A second embodiment of the invention, Fig. 4, comprises an assembly of driven element 27 and parasitic reflector element 28 arranged for operation at two frequencies. The elements are again cut to be naturally effective at the higher frequency and loaded by inductors to be resonant at the lower frequency, the inductors in this construction being in the form of shortcircuited line sections provided by two support arms 29, 30, which join the centres of the two elements and carry short-circuiting bars 31, 33. The open-circuit quarter-wave stubs to short out the inductors at the higher frequency are constructed lengths of twin feeder 32, 34 arranged in the support arms. The assembly is supported from a mast by member 35. The aerial feeder is a coaxial cable, the outer being connected to short-circuit member 31 and the inner being connected through a capacitor 37a to an impedance matching tap on element 27. The arrangement of two support arms shown in this embodiment may be used in Fig. 1. A third embodiment, Fig. 6, comprises a television aerial for use on bands I and III. Hollow tubes 47 are arranged as a half-wavelength dipole resonant at the desired frequency in band III, an inductor 48, wound on a ferrite core, loads the dipole to be resonant at a frequency in band I and is short-circuited at the band III frequency by two quarter-wave stubs 49 housed within tubes 47. Feeder 50 is connected to tapping points on the inductor. A multi-resonant aerial suitable for use as the driven element in Figs. 1 or 4 is shown in Fig. 5. A section 39 halfwavelength long at the highest of three desired frequencies is connected to further sections 41 and 43 through inductors 40 and 42 which allow the sections to become effective at the lower frequencies and increase the length of the aerial appropriately.